The present invention relates to a piezoelectric element having an electro-mechanical conversion function, an ink jet head using the piezoelectric element, an angular sensor, a method for manufacturing the same, and an ink jet recording apparatus including the ink jet head as printing means.
Generally, a piezoelectric material is a material capable of converting a mechanical energy to an electrical energy and vice versa. A typical example of a piezoelectric material is lead zirconate titanate having a perovskite crystalline structure (Pb(Zr,Ti)O3) (hereinafter referred to as “PZT”). In PZT, the greatest piezoelectric displacement is obtained in the <001> direction (the c axis direction) in the case of a tetragonal system, and in the <111> direction in the case of a rhombohedral system. However, many of the piezoelectric materials are polycrystals made up of a collection of crystal grains, and the crystallographic axes of the crystal grains are oriented randomly. Therefore, the spontaneous polarizations Ps are also arranged randomly.
Along with the recent downsizing of electronic appliances, there is a strong demand for reducing the size of piezoelectric elements using a piezoelectric material. In order to meet the demand, more piezoelectric elements are used in the form of thin films whose volumes can be significantly reduced from those of sinters, which have conventionally been used in various applications, and active researches and developments have been made for reducing the thickness of thin-film piezoelectric elements. For example, in the case of tetragonal PZT, the spontaneous polarization Ps is oriented in the c axis direction. Therefore, in order to realize superior piezoelectric characteristics even with a reduced thickness, the c axes of crystal grains forming a PZT thin film need to be aligned vertical to the substrate plane. In order to realize such an alignment, a method as follows has been used in the prior art. On a single crystal substrate made of magnesium oxide (MgO) having an NaCl-type crystalline structure, which has been cut out so that the surface thereof is along the crystal orientation of the (100) plane, a (100)-oriented Pt electrode thin film is formed as a lower electrode on the substrate by a sputtering method, and a PZT thin film having a desirable crystallinity and whose c axis is oriented vertical to the surface of the Pt electrode is formed on the Pt electrode at a temperature of 600 to 700° C. (see, for example, Journal of Applied Physics vol. 65 No. 4 (published on Feb. 15, 1989 from the American Physical Society) pp. 1666–1670, and Japanese Laid-Open Patent Publication No. 10-209517).
It is characteristic of this method that an MgO single-crystal substrate, which makes it possible to realize a piezoelectric thin film that is preferentially oriented in the crystallographic direction in which superior piezoelectric characteristics are exhibited. However, since the MgO single crystal is a very expensive material, it is very costly to mass-produce industrial products using piezoelectric elements including piezoelectric thin films that are formed by this method.
In view of this, various methods have been developed for forming a well-oriented film of a piezoelectric material on an inexpensive substrat substrate. For example, as a method for controlling the plane along which the crystal of a piezoelectric layer such as PZT is preferentially oriented, Japanese Laid-Open Patent Publication No. 2001-88294 discloses a manufacturing method (a sol-gel method) including: forming a base layer whose main component is zirconium oxide on the surface of a substrate; forming a lower electrode containing iridium on the base layer; depositing a very thin titanium layer on the lower electrode; forming an amorphous piezoelectric precursor thin film containing metal element and oxygen element, which forms a ferroelectric that exhibits piezoelectric characteristics, on the titanium layer; and crystallizing the amorphous thin film through a heat treatment at a high temperature, thereby turning the amorphous thin film into a piezoelectric thin film that exhibits a piezoelectric property. It is also disclosed that it is possible to control the crystal orientation of the piezoelectric thin film by controlling the thickness of the titanium layer.
However, while the method disclosed in Japanese Laid-Open Patent Publication No. 2001-88294, supra, is a desirable method that does not use an expensive MgO single-crystal substrate, it is difficult to obtain a well-oriented film having a desirable crystallinity in the film formation process, as in the case of forming a piezoelectric thin film on an MgO single-crystal substrate, because the piezoelectric thin film is formed by a sol-gel method. In view of this, an amorphous piezoelectric precursor thin film is first formed, and then the layered structure including the substrate and the precursor thin film is subjected to a heat treatment in the final step, so that the crystallographic axes are preferentially oriented in a desirable direction.
Thus, when piezoelectric elements are mass-produced with a sol-gel method, the amorphous piezoelectric precursor thin film is likely to be cracked due to changes in the volume during the degreasing step of removing organic substances. Moreover, in the step of heating and crystallizing the amorphous piezoelectric precursor thin film at a high temperature, the film is likely to be cracked or peeled off from the lower electrode due to crystal changes. Furthermore, the heat treatment step after the deposition process adds to the number of steps, whereby the production yield may be reduced.
On the other hand, according to Japanese Laid-Open Patent Publication No. 2001-88294, supra, states that attempts were made to control the orientation of a PZT film, which is a typical ferroelectric thin film, by using a method other than a sol-gel method (including an MOD method) (in which an amorphous thin film is once formed and then the thin film is turned into a crystalline thin film through an aftertreatment such as a heat treatment for crystallization), i.e., by using a method in which a crystalline thin film is directly formed without the crystallization step using a heat treatment, e.g., a sputtering method, a laser ablation method or a CVD method, and that the orientation could not be controlled by any method other than a sol-gel method. The reason is stated to be as follows. The crystallization of the PZT film proceeds gradually from the lower electrode side to the upper electrode side with a sol-gel method, whereas with a CVD method or a sputtering method, the crystallization of the PZT film proceeds randomly, resulting in irregular crystallization, and thus making the orientation control difficult.